Effect of structural differences of carbon nanotubes and graphene based iridium-NHC materials on the hydrogen transfer catalytic activity
Introduction
Technological development has provided us with available new one, two and three-dimensional carbon materials with a specific spatial and geometrical structure [1], [2]. These highly versatile carbon materials are being currently used not only in electronic devices [3], [4], [5], [6] but also as catalyst supports since their graphitic structure makes them inert in many chemical media [7]. For these reasons, in addition to their use as carbocatalysts, where oxidized functionalization is generally required [8], [9], [10], they have been profusely used as supports for hybrid materials having metal oxides, nanoparticles or molecular catalysts as the active sites [7], [11], [12], [13], [14]. These supports were initially evaluated as “innocent” materials due to their uniform hybridization, ordered structure and chemical inertness but it is known that the metal-support interactions are strongly dependent not only on the nature of the metal, but also on the type of carbon material chosen because their electronic [15], [16], thermal [17] or mechanical properties [18], [19] depend on their preparation method, origin and further modifications involving different surface treatments [20], [21].
Carbon nanotubes (CNT) [22] and graphene [2] are the mono- and bi-dimensional long-range ordered versions, respectively, of these carbon materials. The synthetic methods used for carbon oxygen functionalization are well-established procedures. Graphene oxide (GO) is obtained from a severe oxidation process commonly employed for the chemical synthesis of graphene from graphite [23], [24]. The sp2 lattice of the graphene is disrupted upon oxidation by the introduction of oxygen functional groups, predominantly OH and epoxy in the interior of the basal plane while carboxylic acids are formed at the edges, and also where there are morphological defects (carbon atom vacancies) [25]. On the other hand, the oxidation processes, what introduce oxygen functionalization in the nanotubes (namely CNTO), are currently used as a purification step after synthesis [26]. Both oxidized carbon nanomaterials, GO and CNTO, have been utilized previously as catalysts themselves or as catalyst supports by making use of the surface groups in order to immobilize the active species [27], [28], [29], [30]. Thus, not only metallic nanoparticles, but also enzymes or organometallic complexes have been anchored to these oxidized carbon nanomaterials using esterification/amidation reactions with oxygen groups. The activation of carboxylic acids with SOCl2 [31] or carbodiimides [32], and the reactivity of hydroxylic groups towards silanes [33] are routinely employed in order to functionalize CNTO and GO with the catalyst precursors. In general, the catalytic studies conducted on nanohybrid catalyst based on CNT or graphene systems have shown promising results in terms of activity, selectivity, stability and/or cyclability compared to the homogeneous or heterogeneous traditional systems [34].
Recently, our group found a protocol to immobilize iridium N-heterocyclic carbene (NHC) complexes on GO and thermally reduced GO materials by taking advantage of the carbon-based organic chemistry of the surface hydroxylic groups instead of using traditional siloxane chemistry [35]. The hybrid graphene–iridium-NHC materials were found to be efficient hydrogen-transfer catalysts exhibiting good recyclability. The effectiveness of the Ir(I)-NHC complexes as homogeneous catalysts in transfer hydrogenation of unsaturated compounds [36], [37], [38], [39], [40], [41], in particular, those with hemilabile O- and N- donor functions in the NHC carbene ligand [42], [43], is well known in the literature. In addition, an increase in the transfer hydrogenation catalytic activity of Ir-NHC complexes based on hemilabile O- and N- donor functions supported on CNT has been observed [44]. However, to date no systematic comparison of the catalytic activities and selectivities of different carbon-material based catalytic systems has been carried out and, to the best of our knowledge, the present study is unique in trying to establish the possible influence of the structure, surface chemistry and the reconstruction of the aromatic network on the catalytic activity of a hybrid organometallic complex-CNTO/GO carbon nanomaterial system. Despite the similar structure and properties of both materials, the geometrical and spatial differences could give rise to different local environments thereby influencing the catalytic activity.
This work has to tackle two critical points. Firstly, the covalent functionalization of oxidized and thermally reduced carbon nanotubes through surface –OH groups to produce iridium-supported NHC-CNTO complexes comparable to hybrid iridium NHC-GO catalysts has to be performed. Secondly, study the catalytic activity in the transfer hydrogenation of cyclohexanone with 2-propanol to compare with that of hybrid GO catalysts materials. Specific features of the structure and the surface chemistry of the hybrid catalysts have been used in this work to explain the differences in the catalytic performance.
Section snippets
Materials
All the chemicals, including the graphite powder and multiwalled carbon nanotubes, were purchased from Aldrich. Reagent or HPLC grade was employed in all the experiments. Solvents were distilled immediately prior to use with appropriate drying agents or obtained from a Solvent Purification System (Innovative Technologies).
The oxidized CNTs (CNTO) utilized in this work were prepared by acid treatment of commercial bundle-type CNTs, as described previously [45]. The GO utilized in this work was
Preparation and catalytic activity of nanotube-based hybrid catalysts
The nanotube-based hybrid catalysts were prepared following the reaction path depicted in Fig. 1 [35]. The “isolated” hydroxyl groups in the nanotubes react with p-nitrophenylchloroformate, with the subsequent formation of the corresponding p-nitrophenyl carbonate esters [53], [54]. Thereafter, the nucleophilic OH-ending group of the imidazolium salt [MeImH(CH2)3OH]Cl (1) easily displaces the p-nitrophenol group, resulting in the formation of the carbonate intermediates, CNTO-1 and TRCNTO-1.
Conclusions
The protocol for functionalizing oxidized carbon nanomaterials through their surface OH groups on the basis of the activation with p-nitrophenylchloroformate can be successfully applied to oxidized and partially reduced carbon nanotubes in order to immobilize iridium NHC-complexes in their outer and inner walls. The carbon nanotube based iridium-NHC hybrid materials were efficient catalysts for the reduction of cyclohexanone by transfer hydrogenation. These hybrid catalysts have shown a
Acknowledgment
The authors thank the Spanish Ministry of Economy and Competitiveness (MINECO/FEDER) (Projects Consolider Ingenio 2010 CSD2009-00050 and CTQ2013-42532-P), and the Diputación General de Aragón (DGA) (FSE-E07 and FSE-E69) for their financial support. Dr. P. A. thanks MICINN for a Ramón y Cajal contract. J. F-T. and M. B. acknowledge their fellowships from MINECO and MECD (AP2010-0025).
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2022, Comprehensive Organometallic Chemistry IV: Volume 1-15Influence of graphene sheet properties as supports of iridium-based N-heterocyclic carbene hybrid materials for water oxidation electrocatalysis
2020, Journal of Organometallic ChemistryCitation Excerpt :Much more scarce are the studies focused on the graphene properties themselves and the influence of their structural properties on the catalytic performance of the resulting supported hybrid catalytic systems, being most of them centered in studying the effect of graphene sheet reduction. As an example, the positive correlation between hydrogen transfer catalytic activity of iridium N-heterocyclic carbene (NHC) organometallic complexes supported onto GO and TRGO has been recently reported [27]. Bearing this in mind, herein two TRGOs, obtained from two graphites of different crystallinity, have been prepared for the covalent anchorage of an organometallic Ir(I)–NHC complex.
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